In a known bending machine such as a press-brake in which a sheet-like workpiece is bent into a V-shape by pressing with the upper bender (punch) and the lower bender (die), the control amount of the upper bender or lower bender is adjusted by means of a numerical control (NC) system, in accordance with data such as the material and thickness of the workpiece and the conditions of the benders, so that an article having the desired bending angle can be produced.
It is difficult for such a bending machine to adjust the control amount of the bender with high accuracy because of factors such as variations in thickness and the characteristics of the material of the workpiece. In order to solve this problem, there has been proposed a bending machine in which the bending angle of a workpiece is measured by "in-line measurement" during the bending process and based on the measured value, the control amount of the bender is adjusted by feed back control, thereby achieving a higher bending accuracy. In this case, however, it is necessary to measure the springback (return due to elasticity) angle of the workpiece by "in-line measurement" as well, because the springback angle of the workpiece varies depending on the material and thickness of the workpiece, the conditions of the benders and other factors.
One example of the measurement of a springback angle is disclosed in Japanese Patent Publication Laid-Open No. 61-229421 (1986). According to this publication, the springback angle of a workpiece is measured in such a way: After a workpiece has been bent at a desired bending angle .theta..sub.A by bringing the upper and lower benders close to each other, the upper and lower benders are moved apart very slowly. At the time when the pressure imposed on the workpiece becomes zero, the bending angle .theta..sub.B of the workpiece is measured. Then, the springback angle .DELTA..theta..sub.A of the workpiece bent at the angle .theta..sub.A is obtained by substituting the measurement result in the following equation. EQU .DELTA..theta..sub.A =.theta..sub.B -.theta..sub.A
It is generally known that as shown in FIG. 13, a linear relationship (correlation coefficient r=0.96) exists between the springback angle .DELTA..theta. and the bending angle .theta. except for the initial stage of bending, that is, the region where the bending angle .theta. is greater than about 165.degree.. Therefore, by repeatedly measuring, with the above-described method, the springback angle .DELTA..theta. that varies continuously with changes in the bending angle .theta., the relationship between the bending angle .theta. and the springback angle .DELTA..theta. can be easily obtained.
In order to measure the springback angle with the above-described method, it is necessary to release pressure imposed on the workpiece until the value of the imposed pressure becomes zero, in other words, until the upper bender (or lower bender) is moved apart from the workpiece. In actual bending, a bending position is usually located at the end portion of the workpiece 52 as shown in FIG. 14(a), so that the workpiece 52 rises from the lower bender 53 when moving apart the upper bender 51 from the workpiece 52 (see FIG. 14(b)), and the workpiece 52 slips to change the contact point with and the upper bender 51 (see FIG. 14(c)). If pressure is applied to the workpiece 52 again in such an unloaded condition, the relationship between the measured bending angle .theta. and the springback angle .DELTA..theta. will change, resulting in a serious error in the control of the bender.
The invention has been made to overcome the above problems and therefore one of the objects of the invention is to provide a springback angle measuring instrument for V-bending which is capable of accurately detecting the springback angle of a workpiece at a stage where pressure imposed on the workpiece has not been completely released.